1. Field of the Invention
The present invention relates to a method of preparing a group III-V compound semiconductor crystal. Particularly, the present invention relates to a method of preparing a group III-V compound semiconductor crystal in which carbon is doped.
2. Description of the Background Art
Conventionally, there are various prior arts as set forth in the following regarding the method of preparing a group III-V compound semiconductor crystal in which carbon is doped.
In Japanese Patent Laying-Open No. 64-79087 (referred to as "prior art 1" hereinafter), a method of preparing a carbon-doped GaAs single crystal according to the gradient freeze method or horizontal Bridgman method (HB method) is disclosed.
FIG. 6 is a diagram for describing a method of preparing a carbon-doped GaAs single crystal according to prior art 1.
Referring to FIG. 6, a graphite boat 51 as a carbon source is arranged at one side in a quartz ampoule 55. Raw material which is gallium (Ga) 52 is provided in graphite boat 51. Arsenic (As) 57 is provided at the other side in quartz ampoule 55. Quartz ampoule 55 is sealed in vacuum and then installed in an electric furnace to be heated. After the GaAs raw material is synthesized, the temperature is reduced maintaining a constant temperature gradient, whereby a GaAs single crystal is grown.
The carbon of graphite boat 51 reacts with oxygen supplied from As.sub.2 O.sub.3, Ga.sub.2 O and the like remaining in quartz ampoule 55 to result in gas of CO, CO.sub.2 and the like to be doped into the growing GaAs crystal.
It is described that the doping amount of carbon can be controlled according to the total amount of oxygen in the sealed quartz ampoule 55, the synthesization reaction condition, or single crystal growth condition, and the like.
In Journal of the Japanese Association of Crystal Growth, 1991, Vol. 18, No. 4, pp. 88-95 (referred to as "prior art 2" hereinafter), a method of preparing a carbon-doped GaAs single crystal by the vertical gradient freeze method (VGF method) is disclosed.
FIG. 7 is a diagram for describing a method of preparing a carbon-doped GaAs single crystal according to prior art 2.
Referring to FIG. 7, raw material 62 having carbon doped in advance, directly synthesized by the LEC method and boron oxide (B.sub.2 O.sub.3) 64 are provided in a crucible 61 and sealed in vacuum in a quartz ampoule 65. This is installed in a vertical furnace and heated to melt the raw material and boron oxide. By reducing the temperature in the furnace while maintaining a constant temperature gradient, a GaAs single crystal is grown.
Here, boron oxide containing water of 200 ppm spreads around only the periphery of the upper surface of GaAs melt 62. The center area of the upper surface of GaAs melt 62 is exposed to the ambient. According to the method of prior art 2, the upper surface of the melt must be exposed to the ambient to control the stoichiometry of the GaAs melt. The vapor pressure in quartz ampoule 65 is controlled by arsenic 67.
According to this method, the carbon concentration of the crystal depends on the carbon concentration of the raw material.
In U.S. Pat. No. 4,999,082 (referred to as "prior art 3" hereinafter), a method of preparing carbon-doped GaAs single crystal by the vertical Bridgman method (VB method) is disclosed.
FIG. 8 is a diagram for describing a method of preparing carbon-doped GaAs single crystal according to prior art 3.
Referring to FIG. 8, a crucible 71 is filled with GaAs raw material 72. After carbon source 73 is arranged outside of crucible 71, a quartz ampoule 75 is sealed. Quartz ampoule 75 is placed in a vertical furnace and heated to melt the raw material. The furnace is moved upwards while substantially maintaining the set temperature profile. By solidifying the raw material from a seed crystal 77, a GaAs single crystal is grown.
According to this method, carbon source 73 is in fluid communication with compound raw material 72 to allow gas transfer.
Japanese Patent Laying-Open No. 3-252399 (referred to as "prior art 4" hereinafter) discloses a method of preparing a semi-insulating GaAs substrate.
Prior art 4 is characterized in that the impurity which becomes the acceptor is doped so as to result in 1.about.3.times.10.sup.15 atoms/cm.sup.3 after subtracting the concentration of the impurity which becomes the donor in a GaAs crystal.
Japanese Patent Laying-Open No. 2-74597 (referred to as "prior art 5" hereinafter) discloses a chromium-doped semi-insulating GaAs single crystal and a method of preparing thereof. This prior art 5 is characterized in that carbon is contained having a concentration n.sub.c that satisfies both the relations of: EQU 1.times.10.sup.15 cm.sup.-3 .ltoreq.n.sub.c &lt;n.sub.si and n.sub.si -n.sub.c .ltoreq.4.4.times.10.sup.15 cm.sup.-3
for the residual Si concentration of n.sub.si remaining in the single crystal, with the resistivity of at least 10.sup.6 .OMEGA..multidot.cm.
The above-described prior art have various disadvantages. In prior art 1, boron oxide is not used. Therefore, impurity contamination can be expected. Furthermore, since the amount of the carbon source cannot be controlled in this method, it is difficult to control the carbon concentration.
In prior art 2, carbon cannot be doped during the crystal growth since carbon source is not used. There is a problem that the carbon concentration cannot be adjusted during crystal preparation. Furthermore, a part of the carbon in the GaAs melt reacts with oxygen, that is generated as a result of the water in the boron oxide decomposing, to be lost as CO gas. There was a problem that the carbon concentration in the GaAs crystal is lowered.
In prior art 3, it is difficult to control the carbon concentration since the carbon source is located outside the crucible. Furthermore, impurity contamination can be expected since boron oxide is not used.
In prior art 4, carbon is recited as the impurity serving as the acceptor. However, only the doping of zinc and copper is disclosed as the example. There is no description of carbon doping.
Prior art 5 describes a chromium-doped semi-insulating GaAs single crystal containing carbon. However, this prior art 5 is silent about the method of doping carbon.